The present invention generally relates to aircraft propulsion and power systems. More particularly, the invention relates to apparatus and methods by which propulsion engines may be operated to maintain a balance between propulsion requirements and delivery of power to meet secondary power (non-propulsive power) requirements within the aircraft.
A typical commercial aircraft may be propelled with turbine engines. One or more of the engines of such an aircraft may be provided with secondary power-takeoff shafts that may be coupled with an auxiliary gearbox to drive electrical generators and hydraulic pumps during flight. Additionally, some aircraft engines may deliver bleed air to drive an environmental control system (ECS) of the aircraft so that proper cabin pressurization and temperature control may be maintained.
An aircraft turbine engine may be controlled to rotate at various speeds in order to satisfy various propulsion requirements. During take-off and climbing, thrust requirements may be high and engine speeds may be correspondingly high. During descent, thrust requirements may be low and engine speed may be correspondingly low. If the engine were used exclusively for providing thrust, then engine speed, during low thrust requirement periods could be lowered to a rate that just exceeds surge or stall conditions for the engine. However, because a typical engine drives secondary loads through a power takeoff shaft and/or through extracted bleed air, the engine speed must be maintained at a level that may accommodate the secondary loading even when thrust requirements may be virtually non-existent, e.g., during descent. This may result in a need to maintain a higher engine speed than that which is needed to supply thrust so that surge conditions do not develop in the engine. Balancing between a low thrust requirement and a higher than necessary engine power output is typically achieved by opening bleed air valves to reduce undesired pressure in the engine while maintaining the engine in a non-surge state. When bleed valves are opened, energy from the engine is effectively discharged to the atmosphere and thus becomes wasted energy.
A typical electronic engine controller (EEC) may be programmed so that a maximum amount of secondary loading is established as a basis for determining how much power may be needed from the engine during low thrust requirements. In other words, a worst-case secondary power requirement is programmed into the EEC. The bleed valves may be opened on the basis of signals from an EEC that is so programmed.
While such a system may assure that secondary loading does not produce surge conditions in the engine, it may nevertheless be energy wasteful. If actual secondary loading is lower than the worst-case condition, then engine surge could be precluded with a reduced amount of bleed air expulsion from the engine. But it has heretofore been impracticable to determine actual or real-time secondary power consumption in a manner that would allow an EEC to safely control bleed air valve opening on a real-time basis.
As can be seen, there is a need for a system by which secondary loading of an aircraft engine may be accurately determined on a real-time basis so that an EEC may control bleed air valve opening as a function of the secondary loading and thereby maintain a desired surge margin without excessive bleed air expulsion from the engine.